SpaceX's Falcon 9 rocket has already proved itself able to get a cargo payload to the International Space Station. Orbital Sciences' Antares rocket, set for its first test launch Wednesday evening, is a very different animal.

Glistening white and standing 130 feet tall, the second of two commercial rockets NASA is relying on to ferry cargo to the International Space Station is set for its first test flight at 5 p.m. EDT Wednesday.

If all goes well, the Antares rocket, built and operated by Orbital Sciences Corp., will join Space Exploration Technologies Corp.'s (SpaceX) Falcon 9 as replacements for the space shuttles that carried cargo to and from the ISS. The Falcon 9 and its Dragon cargo capsule already have completed two formal resupply missions to the station.

"We did come late to this activity," acknowledges Frank Culberston, a former NASA astronaut and now an executive vice president at Orbital Sciences. The company signed on to NASA's Commercial Orbital Transportation Services (COTS) program about 1-1/2 years after the program began, after NASA dropped one of the two initial participants.

NASA's initial choice of two companies for the COTS program involved aerospace upstarts born within a year of each other. Now, the program has paired a grizzled spaceflight veteran with a precocious tweenager.

Orbital Sciences has been building satellites and building and launching rockets for more than 30 years. SpaceX was founded in 2002.

Their approaches to designing and building their rockets are markedly different.

SpaceX has prided itself on designing and building its hardware in-house, beginning with its two-stage, liquid-fueled Falcon 1 rocket. The Falcon 1, with a single engine in its first stage, had teething problems. First launched in March 2006, the rocket's initial three missions failed to deliver anything to orbit. After a fourth, successful test that put a dummy payload into low-earth orbit, the Falcon 1 went on to loft a Malaysian remote-sensing satellite a year later, only to be retired as SpaceX focused on its workhorse, the Falcon 9.

Nearly 200 feet tall, the Falcon 9 boasts nine main engines and can loft much heavier payloads than its predecessor. Sometime this year, the company plans to test a third rocket, the Falcon Heavy – in effect three Falcon 9s strapped side by side, with the center booster topped with a second stage and a payload. The company bills the Falcon Heavy as the most powerful rocket the US has seen since the Apollo program's Saturn V rocket in the 1960s and '70s.

The company's ultimate quest is to develop reusable rockets, in addition to the reusable capsule it has built to carry cargo and eventually humans. SpaceX is testing a propulsion system that will allow a rocket booster to land vertically.

Last month, the propulsion system on a four-legged test frame lofted the frame and a dummy rocket 263 feet in the air, held that height for just over 30 seconds, then gradually eased back to Earth for a landing. At launch, a full-size manikin dressed to look like Johnny Cash stood on the frame and survived the round trip standing up.

Orbital Sciences, on the other hand, has built its Antares rocket largely from hardware that already has been space tested, explains Lennard Fisk, a professor of space science at the University of Michigan at Ann Arbor and a member of Orbital Sciences' board of directors.

The company turned to Ukraine's Yuzhnoye Design Bureau to help with the tanks, pumps, and plumbing needed to support Antares's two liquid-fuel motors – themselves built in the former Soviet Union but modified for Antares by a US rocket-motor company. Yuzhnoye builds Russia's Zenit rockets.

The second stage is powered by a solid-fuel motor built by the company that produced the solid-fuel boosters for the space shuttle.

Orbital Sciences' progress has not been without hiccups.

During launches for NASA in 2009 and 2011, the sleek shell, or faring, that the company uses to protect a payload while the rocket rises through the atmosphere failed to split open, preventing the satellites from reaching orbit. It has since modified the faring, which encase a dummy payload perched atop Antares.

In addition, during a test in 2011, one of the engines caught fire during a test. The US company that modified them for Orbital Sciences, Aerojet, repaired the defect. The motor has passed subsequent "hot fire" tests in January and February.

The program also experienced delays as technicians coped with the challenges of installing and testing new hardware designed to handle frigid liquid fuel at the Mid Atlantic Regional Spaceport on Wallops Island in Virginia. Up to this point, the spaceport and the adjacent Wallops Island Fight Center, which NASA runs, had lofted solid-fuel rockets.

Today, NASA and Orbital will test the fruits of their labor.

"When we first started this program six years ago, there was no launch pad and no rocket" at the spaceport, said Phil McAlister, NASA's commercial-spaceflight development director during a prelaunch briefing. "I'm very much looking forward to the day where we have regular cargo resupply runs to the International Space Station. Hopefully we'll see that very soon."